Strategies for the selective delivery of small molecule cancer chemotherapeutic agents to tumor cells (e.g. antibody-drug conjugates) hold promise as a way to, in effect, increase their therapeutic index (Chari, R. V. J. et al. 2014). A requirement for the drug in many of these approaches is a validated linker strategy (Ducry, L. et al. 2010), the most critical component of which is the identification of a site on the drug that may be modified without interfering with its ability to access and bind to its target receptor and express its activity. More broadly, the identification of such modifiable sites on bioactive natural products can facilitate chemical biology and mechanism of action studies and enable exploration of more novel linked constructs.
Non-aromatic polyketide natural products are a pharmaceutically important class of compounds due to their often high levels of biological activity. A number of these compounds have been identified as potent antitumor agents and have thus been highly pursued for use as therapeutic agents. By binding to tubulin, the molecules disrupt microtubule dynamics, inhibiting mitosis and leading to cell death. Due to limited access to some of these important natural products or a need for innate structural modifications to improve their pharmacological properties, the use of synthetic chemistry has become paramount for the further development of a number of polyketides as therapeutic agents.
The epothilones are a family of cytotoxic natural products that were first isolated from the myxobacterium Sorangium cellulosum in 1987 (Hofle, G. et al. 1996; Gerth, K. et al. 1996). While initially of some interest for their antifungal properties, these compounds attracted much more attention from the scientific community in 1993 when they were found to exhibit potent taxane-like antitumor activity (Bollag, D. et al. 1995).
Dictyostatin (Petit, G. R. et al. 1994; Isbrucker, R. A. et al. 2003; Paterson, I. et al. 2004), for which we recently reported a synthesis that proceeds in just 14 steps in the longest linear sequence (Ho, S. et al. 2013), is a worthy candidate for linker strategy validation in that it is among the most potent of the microtubule-stabilizing agents (MSAs) known to bind to the taxane binding site on the β-tubulin subunits of microtubules, retains significant potency against several taxane-resistant cell lines, and has recently been shown to be a rare example of a brain-penetrant MSA (Brunden, K. R. et al. 2013).
Epothilones A (Epo A) and B (Epo B), have been found to act via the same microtubule-stabilizing mechanism of action as taxol, the first therapeutic agent with this mechanism to obtain FDA approval (FIG. 1). While Epo A exhibits similar activity to taxol in a number of cancer cell lines, Epo B is about tenfold more potent in the same cell lines (Altmann, K.-H. et al. 2007). Though they act at the same microtubule binding site, the epothilones are significantly more active than taxol for inhibiting the growth of multidrug-resistant (MDR) cancer cell lines. In taxol-resistant cancer cell lines that overexpress phosphoglycoprotein 170 (P-gp), epothilones A and B are able to maintain almost full anti-proliferative activity because they are poor substrates for the P-gp efflux pump (Altmann, K.-H. et al. 2000). Epothilones have also been shown to retain activity in cancer cell lines that have developed taxol-resistance due to particular tubulin mutations, which is the another main mechanism of taxol-resistance (Giannakakou, P. et al. 1997).
Besides activity, the epothilones have the practical advantage of exhibiting increased solubility relative to taxol, meaning they would not require clinical formulation vehicles such as Cremophor which has been implicated for some of taxol's clinical side effects (Rowinsky, E. K. 1997). Due to these advantages over taxol, the epothilones have become highly attractive targets for drug discovery efforts and total synthesis efforts. Over 30 total syntheses of Epo A and B have been reported, as well as extensive studies of the structure-activity relationship (SAR) of the epothilones. As a result of these efforts, a number of epothilone-derived compounds have been advanced to clinical trials as potential anticancer drugs (Nicolaou, K. C. et al. 1998; Harris, C. et al. 1999; Nicolaou, K. C. et al. 2001).
Dictyostatin (Petit, G. R. et al. 1994; Isbrucker, R. A. et al. 2003; Paterson, I. et al. 2004)), for which we recently reported a synthesis that proceeds in just 14 steps in the longest linear sequence (Ho, S. et al. 2013), is a worthy candidate for linker strategy validation in that it is among the most potent of the microtubule-stabilizing agents (MSAs) known to bind to the taxane binding site on the β-tubulin subunits of microtubules, retains significant potency against several taxane-resistant cell lines, and has recently been shown to be a rare example of a brain-penetrant MSA (Brunden, K. R. et al. 2013).